The present invention generally relates to thermal transfer type printing apparatuses, and more particularly to a thermal transfer type printing apparatus which prints a color picture or the like with a desired gradation (tone).
Recently, it is common to obtain a digital signal by subjecting signals such as a color video signal and a color graphic signal which is obtained from a personal computer to an analog-to-digital (A/D) conversion, and supply the digital signal to a printing apparatus so as to print a color picture or graphics on a printing paper. Printing apparatuses may be roughly divided into impact type printing apparatuses and non-impact type printing apparatuses. An impact type printing apparatus generally comprises a head having a plurality of wire-dots, and the color picture or the like is printed on the printing paper when the wire-dots strike an ink ribbon or one of ink sheets placed on top of the printing paper. The ink ribbon is coated with ink of three colors, and the ink sheets are each coated with ink of one of the three colors. However, in the impact type printing apparatus, a large mechanical noise is generated during the printing operation due to the striking action of the wire-dots. Further, there is a disadvantage in that the resolution of the printed color picture or the like is low.
On the other hand, non-impact type printing apparatus may be roughly divided into ink-jet type printing apparatuses and thermal transfer type printing apparatuses. An ink-jet type printing apparatus generally comprises a head having a plurality of nozzles, and the color picture or the like is printed on the printing paper when ink of three colors, for example, is sprayed onto the printing paper from the nozzles. Compared to the impact type printing apparatus, the ink-jet type printing apparatus is advantageous in that only a small mechanical noise is generated during the printing operation. However, there is a disadvantage in that the nozzles are easily blocked by refuse of ink or the like which adheres to the nozzles.
A thermal transfer type printing apparatus comprises a thermal printing head. The color picture or the like is printed on the printing paper when the thermal printing head heats a single ink ribbon, one of ink ribbons, or one of ink films placed on top of the printing paper. The single ink ribbon is coated with ink of three colors at predetermined parts thereof, for example, the ink ribbons are each coated with ink of one of the three colors, and the ink sheets are each coated with ink of one of the three colors. Compared to the impact type printing apparatus, the thermal transfer type printing apparatus is advantageous in that only a small mechanical noise is generated during the printing operation. Moreover, the problem of the refuse of ink adhering on the nozzles and blocking the nozzles as in the case of the ink-jet type printing apparatus, will not occur in the thermal transfer type printing apparatus. However, according to the thermal transfer type printing apparatus, the thermal printing head first prints the entire picture or the like on the printing paper by using an ink film (a part of the single ink ribbon, or an ink ribbon) which is coated with ink of a first color among ink films (parts of the single ink ribbon, or ink ribbons) coated with ink of three colors, for example. Next, the thermal printing head prints the entire picture or the like on the printing paper which is already printed by use of the first color ink, by using an ink film which is coated with ink of a second color. Similarly, the thermal printing head prints the entire picture or the like on the printing paper which is already printed by use of the first and second color ink, by using an ink film which is coated with ink of a third color. In other words, the thermal printing head prints the entire picture or the like by only using one ink sheet at a time, that is, by only using ink of one color at a time. For this reason, when the thermal transfer type printing apparatus uses ink sheets which are respectively coated with ink of three colors, such a printing operation which employs one of the ink sheets must be performed three times.
Accordingly, when printing the color picture or the like in the thermal transfer type printing apparatus, the following operations (a) through (c) must be performed in order to print the entire picture or the like on the printing paper which is already printed by use of the first color ink, by using the ink film which is coated with the second color ink.
(a) Change the ink sheet which is coated with the first color ink with the ink sheet which is coated with the second color ink.
(b) Move and return the printing paper which is already printed by use of the first color ink back to an original position where the printing operation which uses the first color ink was started.
(c) Return the thermal printing head back to an original printing start position.
Operations similar to the above operations (a) through (c) are also performed when the thermal printing head prints the entire picture or the like on the printing paper which is already coated with the first and second color ink, by using the ink film which is coated with the third color ink. Hence, the thermal transfer type printing apparatus must be provided with a mechanism for changing an ink film with another ink film which is coated with ink of another color, and a mechanism for moving and returning the printing paper back to the original position. As a result, the cost of the thermal transfer type printing apparatus becomes high. In addition, the mechanism for moving and returning the printing paper back to the original position, must move and return the printing paper with a high positional accuracy. However, in actual practice, a slight error is introduced when the mechanism moves and returns the printing paper back to the original position, and errors are accordingly introduced in the printing positions. When such errors are introduced in the printing positions, it is not possible to obtain sharp contours of images in the printed color picture. Further, when the thermal transfer type printing apparatus uses ink sheets which are respectively coated with ink of three colors, a printing operation which employs one of the ink sheets must be performed with respect to the printing paper three times, and there is a problem in that it takes a long time to print a color picture or the like.
On the other hand, there are basically two methods of printing the picture or the like with a desired gradation (tone) in the thermal transfer type printing apparatus, and the two methods are determined by the kind of thermal printing head which is employed in the thermal transfer type printing apparatus.
A thermal printing head which comprises a heating dot element having an area which is the same as the area of one picture element, is used according to a first method of printing with the desired gradation. According to this first method, the heating area of the heating dot element is controlled by controlling the voltage or the like which is applied to the heating dot element, so as to print one picture element with the desired gradation. However, it is extremely difficult to linearly control the heating area of the heating dot element. For this reason, it is extremely difficult to print the picture or the like with the desired gradation by use of this conventional thermal printing head.
A thermal printing head which comprises a plurality of heating dot elements with respect to the area of one picture element, is used according to a second method of printing with the desired gradation. According to this second method, the heating dot elements are selectively heated so as to print one picture element with the desired gradation. When printing one picture element with one gradation out of 64 possible gradations, for example, one picture element is described by a maximum of an 8.times.8 matrix of dots. In this case, it is necessary to employ a thermal printing head having 8 heating dot elements arranged in a vertical row, and the printing with respect to one picture element is completed when the 8 heating dot elements are selectively heated as the vertical row of 8 heating dot elements move in the horizontal direction by a distance of 8 heating dot elements. Accordingly, in the thermal printing head which is used in the second method, it is necessary to provide 8 times the number of heating dot elements required in the aforementioned thermal printing head which controls the heating area of the heating dot element, and the manufacturing cost of the printing apparatus becomes high. Further, since the printing with respect to one picture element is carried out while the thermal printing head moves in the horizontal direction by a distance of 8 heating dot elements, the time it takes to print one picture element is 8 times longer than the time required in the aforementioned thermal printing head which controls the heating area of the heating dot element.